Paranasal sinuses are four pairs of air-filled cavities connecting to the nasal passage. The paranasal sinuses are named after the cranial bones in which they are located: the frontal sinuses, the maxillary sinuses, the ethmoid sinuses, and the sphenoid sinuses. A membrane lining the paranasal sinuses secretes mucus, which drains into the nasal passage through a small channel in each sinus. Healthy sinuses are sterile and contain no bacteria. In contrast, the nasal passage normally contains many bacteria that enter through the nostrils as a person breathes.
A number of factors and processes are involved in maintaining healthy sinuses. The mucus secreted by the membrane lining must be fluid but sticky, in order to flow freely yet absorb pollutants and entrap bacteria. It must also contain sufficient amounts of bacteria-fighting substances such as antibodies. Additionally, small hair-like projections called cilia, located in the nostril, must beat in unison to propel mucus outward, in order to expel bacteria and other particles. Moreover, the mucous membranes themselves must be intact, and the sinus passages must be open to allow drainage and the circulation of air through the nasal passage. When one or more of these processes or factors are amiss, causing obstruction of the sinus passage, an infection called sinusitis develops.
Sinusitis is an inflammation of the mucous membrane lining one or more paranasal sinuses. Rhinitis is an inflammation of the mucous membrane lining the nasal passage. Rhinitis and sinusitis usually coexist and are concurrent in most individuals; thus most guidelines and experts now have adopted the term rhinosinusitis (Fokkens et al., Rhinology 2012 March; 50 (Suppl 23): S5).
The symptoms of rhinosinusitis include nasal congestion and obstruction, colored nasal discharge, anterior or posterior nasal drip. Subjects may also experience facial pain or pressure, and in severe cases, suffer a reduction or a loss of smell (Fokkens et al., 2012). There are two different types of rhinosinusitis: acute and chronic. Acute rhinosinusitis is characterized as rhinosinusitis with complete resolution of symptoms within 12 weeks, while chronic rhinosinusitis lasts longer than 12 weeks, and usually involves tissue damage (Fokkens et al., 2012). Nasal polyps are frequently present in some subjects with chronic rhinosinusitis based on epidemiologic studies.